1. Field of the Invention
The present invention relates to a pixel structure and a method for generating drive voltages in the pixel structure; more specifically, the present invention relates to a pixel structure and a method for generating drive voltages according to com-voltage signals in the pixel structure.
2. Descriptions of the Related Art
In recent years, flat panel displays (FPDs) have developed rapidly and gradually replaced traditional cathode radiation tube displays. Today, major flat panel displays include: organic light-emitting diodes displays (OLEDs), plasma display panels (PDPs), liquid crystal displays (LCDs), and field emission displays (FEDs). Each of these FPDs is composed of many pixels, each of which is a key component of the FPD.
An LCD is one kind of the FPDs that has high resolution, small size, and low power consumption. Furthermore, the LCD has better performance, higher productivity, and lower prices compared to the other FPDs. As a result, the market sale of LCD has increased.
In a conventional LCD, each pixel needs a drive voltage for providing an electric field for liquid crystal reorientation in the pixel, such that the LCD can display a frame with different brightness and contrast on different pixels. Because of the single drive voltage applied to each pixel in the conventional LCD, the color will be inversed at a large visual angle and degrade display performance. Furthermore, in conventional twisted nematic liquid crystal displays (TN LCDs), there are problematic gray level inversions caused by the over-changing of the visual angle. In general, for LCDs, a higher gray level in a pixel indicates a higher level of brightness in the pixel. For example, a pixel with a gray level 0 displays black, while one with a gray level 255 displays white. However, when viewing the TN LCD at a large visual angle, pixels of the lower gray level display higher brightness than those of the higher gray level. Hence, the user views the display with black-white inversion, also known as gray level inversions.
To reduce the above drawbacks, some systems and methods capable of driving different sub-pixels using different drive voltages within a signal pixel have been developed. Multiple drive voltages are required in each of the pixels to drive different sub-pixels. Therefore, multiple com-lines are needed in each pixel for transmitting the multiple drive voltages. In other words, when there are two drive voltages required for driving two sub-pixels in a pixel, two com-lines are fabricated for the pixel. When there are three drive voltages required for driving three sub-pixels in a pixel, three com-lines are fabricated for that pixel. The more numbers of drive voltages required within a pixel for driving multiple sub-pixels, the more corresponding com-lines needed.
As shown in FIG. 1, a conventional pixel structure 1 of the LCD of the prior art comprises a first sub-pixel area 101, a second sub-pixel area 103, a third sub-pixel area 105, a first com-line 107, a second com-line 109, a third com-line 111, gate lines 113a and 113b, thin-film transistors (TFTs) 115, 117 and 119, and data lines 121a and 121b. The on/off state of the TFT 115 and the operation of the first sub-pixel area 101 are controlled by a gate voltage transmitted by the gate line 113a. The on/off state of the TFT 117 and the operation of the second sub-pixel area 103 are also controlled by the gate voltage transmitted by the gate line 113a. Moreover, the on/off state of the TFT 119 and the third sub-pixel area 105 are controlled by the gate voltage transmitted by the gate line 113a. The data line 121a transmits drive voltages required by the first sub-pixel area 101, the second sub-pixel area 103 and the third sub-pixel area 105 via the TFTs 115, 117 and 119, respectively. The first com-line 107 is configured to transmit the drive voltage required by the first sub-pixel area 101, while the second com-line 109 is configured to transmit the drive voltage required by the second sub-pixel area 103. Similarly, the third com-line 111 is configured to transmit the drive voltage required by the third sub-pixel area 105.
FIG. 2 is a schematic diagram illustrating voltage waveforms in the conventional pixel structure, which includes: voltage waveforms of the first sub-pixel area 101, the second sub-pixel area 103, the third sub-pixel area 105, the first com-line 107, the second com-line 109, the third com-line 111, and the gate line 113. In FIGS. 1 and 2, there are three com-lines 107, 109, and 111 required for transmitting three different drive voltages to the sub-pixel areas 101, 103, and 105.
Even though providing multiple drive voltages through multiple com-lines in a single pixel of the prior art may reduce color washout and gray level inversion at large visual angles, the increased number of com-lines results in an increased metal area. As a result, the aperture ratio of the pixels is decreased. Moreover, the increased in com-line signals also increases the complexity and cost of the pixels' peripheral wiring.
Thus, it is important to find a method for decreasing the number of com-lines to increase aperture ratio of the pixel and to reduce the complexity and cost of wiring while preserving the benefits of having multiple drive voltages.